The present invention relates to a spectroscopic method for measuring absorption spectrum of a gas to be measured (hereinafter, xe2x80x9csample gasxe2x80x9d) using laser diode beam, for identifying impurities (hereinafter, also referred to as xe2x80x9cobjects (to be measured)xe2x80x9d) in the gas and for analyzing abundance ratio and concentration of the identified objects from the absorption spectrum with high precision and high sensitivity. Particularly the present invention relates to a spectroscopic method for analysing impurities in the gas, which includes main ingredient and impurities, with high precision and high sensitivity where it is difficult to take an absorption spectrum of the impurities due to the interference absorption of the main ingredient because the absorption spectra of the main ingredient and the impurities are in similar wavelength range.
As a conventional spectroscopic method for analysing a small quantity of impurities in a gas with relatively good precision and sensitivity, a spectroscopic analysis method for measuring absorbance of laser diode has been used. However, it is very difficult to analyse a gas with high precision and sensitivity in which the absorption spectrum of the object to be measured is overlapped on the spectrum of the main ingredient, and the quantity of the object is in PPB(1/109) level, as in the case of absorption spectra of ammonia NH3 and water vapor H2O, shown in FIG. 6. For example, it is very difficult to analyse the gas of which the object to be measured is water vapor and the main ingredient is a polyatomic molecule, such as ammonia NH3 (or silane SiH4).
Therefore, in order to analyse a gas including a main ingredient and impurities, such as water vapor, the inventors of the present invention have developed and filed a Japanese patent application No. HEI09-91158 of xe2x80x9cAn apparatus and a method for spectroscopic analysis using dual cell systemxe2x80x9d. A schematic diagram of an embodiment of the dual cell system is shown in FIG. 7, where the apparatus comprises a sample cell 52 into which a sample gas G is introduced and a cancel cell 53 into which a cancel gas C, consisted of the main ingredient of the sample gas G without the impurities in the sample gas G, is introduced.
According to the above method and apparatus shown in FIG. 7, a laser beam L from a laser source 51 is splitted into laser beams L1 and L2 by the beam splitter 54, and the optical characteristics are controlled to be identical with each other. Then, the splitted beams L1 and L2 are respectively introduced into the sample cell 52 and the cancel cell 53. Then, the transmitted beams L1t and L2t are respectively detected and photoelectrically converted into signals by the detectors 55 and 56. Then the converted signals are respectively send to lock-in amplifiers 57 and 58. The second-order differential spectra of the transmitted beams L1t and L2t are obtained by the lock-in amplifiers 57 and 58 respectively, and the second derivative spectra are respectively digitized by the AD converters 59 and 60 and then input to a computer 61. Then, the absorption spectrum of the cancel gas C is subtracted from the absorption spectrum of the sample gas G by processing the input signals with pre-memorized information, such as calculating formulae, and an absorption spectrum of the objects(impurities) can be taken. According to the above described method, it is possible to analyse impurities in the sample gas G with high precision and sensitivity. A controller for laser diode 51a, a display 62 connected to the computer 61, gas inlets 63 and 64 for the sample gas G and the cancel gas C are also shown.
However, according to the above described conventional spectroscopic method, it is important to make configurations and specifications of the sample cell 52 and the cancel cell 53 identical, and to make the two optical systems be operated under identical conditions. Thus, the cost for producing the apparatus for performing the conventional method becomes very high because of the necessary parts and components.
Therefore, there has been a strong need to develop a spectroscopic analysis method with high precision and sensitivity performed in a more compact and simple apparatus which uses a single cell system, where the single cell is once used as a sample cell for a sample gas G and then used as a cancel cell for a cancel gas C without using the conventional dual cell system having two identical cells of identical specifications and two identical optical systems operated under identical operating conditions.
When a spectroscopic analysis is performed using a single cell system by taking absorption spectra of the sample gas and the cancel gas, it is very difficult to exactly match the absorption spectra in order to calculate subtraction of the absorption spectrum of the cancel gas from that of the sample gas because the spectra are not taken at the same time but separately taken one after another.
This is because the emitting wavelength of the laser is changed due to an extremely minute change in the temperature of the laser diode while exchanging the sample gas with the cancel gas into the single cell. Further, the emitting wavelength of the laser is also changed due to the variations in the environment, such as changes in surrounding temperature, or the limitations in the effective controllable resolution of control devices between the moments of taking absorption spectra of the sample gas and the cancel gas.
Therefore, according to the spectroscopic analysis of a sample gas using the single cell system, it is very difficult to analyse objects(impurities) with high precision and sensitivity because the subtraction of the two spectra could not be exactly performed due to the difficulty in exactly matching the absorption spectra of the sample gas and the cancel gas.
In order to overcome this problem, it is known in the art to lock the wavelength of the laser diode based on the absorption spectrum of water vapor, but the measurement precision of this method is not better than that of the method using the dual cell system.
Further, to use the wavelength of the absorption spectrum of water vapor as a reference wavelength is also used in the conventional spectroscopic analysis using the single cell system. According to this method, after making the wavelength of the absorption spectrum of water vapor as a reference wavelength and marking the value of [driving current of emitting the laser diode (mA)]xc2x7[wavelength (nm)], the laser beam is drived based on this reference wavelength and the marked value, and an absorption spectrum of the object to be measured is taken from the laser beam transmitted through the single cell. However, even by this method, it is still impossible to exactly reproduce the spectra to have the same wavelength scale even by the same driving current at every time because of the minute change in the environment, such as change in surrounding temperature.
Therefore, there has been a strong need to achieve a spectroscopic analysis of high precision with a simple apparatus using single cell system.
Therefore, the present invention has been made in view of the above mentioned problem, and instead of using the dual cell system which needs precision and expensive parts and comprises two cells(a sample cell and a cancel cell) of identical specifications and dimensions, it is an object of the present invention to provide a spectroscopic method for analysing objects(impurities) in a sample gas comprising a main ingredient and the objects, both of which the absorption spectra exist in the same wavelength range, with high precision and sensitivity by using a compact and simple single cell system which uses a single cell as both of a sample cell and a cancel cell.
In accordance with an aspect of the present invention, there is disclosed a spectroscopic method for analysing objects in a sample gas using a laser beam comprising: i) a step of splitting a laser beam into a first laser beam and a second laser beam; ii) a step of transmitting said first laser beam into a sample cell where a sample gas is introduced, and measuring an intensity of a spectrum of said transmitted first laser beam ; iii) a step, being performed while performing said step ii), of transmitting said second laser beam into a reference cell where a reference gas is introduced, and measuring an intensity of a spectrum of said transmitted second laser beam, wherein said reference gas comprises an ingredient having at least two spectral lines of which wavelengths in an absorption spectrum of said reference gas are already known; and iv) a step of identifying a wavelength of objects to be measured in said sample gas by comparing said spectrum of sample gas with said spectrum of reference gas using said at least two spectral lines of said reference gas as reference wavelengths.
In accordance with another aspect of the present invention, there is disclosed a spectroscopic method for analysing objects in a sample gas using a laser beam comprising: i) a step of splitting a laser beam into a first laser beam and a second laser beam; ii) a step of transmitting said first laser beam into a sample cell where a sample gas is introduced and taking an absorption spectrum of said transmitted first laser beam, wherein said sample gas comprises a main ingredient and objects to be measured; iii) a step, being performed while performing said step ii), of transmitting said second laser beam into a reference cell where a reference gas is introduced and taking a first absorption spectrum of said transmitted second laser beam, wherein said reference gas comprises an ingredient having at least two spectral lines of which wavelengths in said first absorption spectrum are already known; iv) a step, after exhausting said sample gas out of said sample cell and introducing a cancel gas into said sample cell, of transmitting said first laser beam into said sample cell and taking an absorption spectrum of said transmitted first laser beam, wherein said cancel gas comprises only said main ingredient of said sample gas without said objects; v) a step, being performed while performing said step iv), of transmitting said second laser beam into said reference cell and taking a second absorption spectrum of said transmitted second laser beam; vi) a step of making scales of wavelength axes of all of said spectra the same by comparing a first set of absorption spectra with a second set of absorption spectra, wherein said first set comprises said absorption spectrum of said sample gas and said first absorption spectrum of said reference gas, said second set comprises said absorption spectrum of said cancel gas and said second absorption spectrum of said reference gas, and said known spectral lines of first and second spectra of said reference gas are used as references; and vii) a step of taking an absorption spectrum of said objects to be measured by subtracting said spectrum of said cancel gas from said spectrum of said sample gas, said scales of wavelength axes of said spectra of cancel gas and sample gas being made the same in said step vi).